Most oil well systems utilize a rotary drill bit for drilling a subterranean well. This system utilizes a column of drill pipe that is attached to a multi-faceted drill bit which rotates as it is forced downward through the soil and rock. The drill bit cuts into the soil and rock and this creates ground cuttings and debris that accumulate in the drilling container. Hence, a drilling fluid must be employed to carry the excavated cuttings to the surface for removal. This is necessary to prevent jamming and clogging of the drill bit and pipe and to keep the bottom of the hole clean at all times.
To remove cuttings produced during an oil well drilling operation, a drilling fluid is circulated down the drill pipe and up the annulus between the drill pipe and the formation whereby the cuttings are carried to the surface. The cuttings are then shaken out of the drilling fluid and the fluid is recirculated. In addition to removing the cuttings, the drilling fluid must also coat and lubricate the drill bit and stabilize the shale formation.
In some areas, formations known as heaving or sloughing shales are penetrated during a well-drilling operation. These shales have a tendency to swell or crack upon contact with the drilling fluid whereupon the bore hole walls are rendered unstable and the heaving shale material, which makes up such walls, sloughs and caves into the bore hole. Sloughing of shale material into the bore hole can cause the drill stem to become stuck and can enlarge the bore hole with the result that large subterranean cavities are formed. Furthermore, where sloughing occurs while the drilling bit is being changed at the surface, the bore hole fills up and must be cleared before drilling can proceed. In addition, the heaving shale material taken up into the drilling fluid adversely affects the viscosity characteristics of the drilling fluid to the point where the fluid must be chemically treated to reduce the viscosity thereof or it must be diluted followed by the addition of weighing material to maintain a given mud weight.
This drilling fluid then not only must carry drill cuttings to the surface of the well but must also control formation pressure and maintain stability of the surrounding rock and soil formation in the uncased sections of the bore hole. They must also protect productive formations as well as cool and lubricate the drill bit and drill string. Hence, a drill fluid must have certain chemical and physical characteristics such as low fluid loss or filtration control, solid suspension power, shale inhibition, insensitivity to shear, thermal stability and the like.
A wide variety of additives have been utilized in drilling fluids to stabilize shale formations encountered during oil well drilling operations. For example, Weiss et. al. U.S. Pat. No. 2,802,783 discloses saturating an aqueous drilling fluid with calcium hydroxide; Scott et. al. U.S. Pat. No. 3,017,351 discloses incorporating into the drilling fluid an acetate salt of an amide of a fatty acid and a polyamine along with ammonium sulfate, oil and an acrylamide polymer hydrolyte; Blattel et. al. U.S. Pat. No. 4,341,645 discloses slats of copolymers of acrylic acid and hydroxypropyl acrylate; while Dupre et. al. U.S. Pat. No., 4,299,710 discloses the combination of an acid-containing polymer and a polysaccharide wherein the acid-containing polymer can be a copolymer of any one of a long list of saturated carboxylic acids, including acrylic acid on a half-amide of a dicarboxylic acid, with any one of a long list of vinyl monomers, including 2-hydroxyethyl and hydroxypropyl acrylates. These additives all require the formation of salts in order to be utilized in aqueous-based drilling fluids. The potassium chloride salts of polyacrylamide and modified starch have also bene used as shale stabilizer compositions in drilling mud for water sensitive shales in wells Clark et. al. J. Petrol Tech. pp 719-727 (June, 1976).
Polygalactomannans and their derivatives are well known compositions which have many uses as thickening agents in aqueous systems. They are polysaccharides composed principally of galactose and mannose units and are usually found in the endosperm of leguminous seeds, such as guar, locust bean, honey locust, flame tree and the like. Guar gum, for example, is composed mostly of a polygalactomannan which essentially is a straight chain polymannan with single-membered galactose branches. The ratio of galactose to mannose units in the guar polymer is 1:2.
Locust bean gum is a polygalactomannan of similar molecular structure in which the ratio of galactose to mannose is 1:4. Guar and locust bean gums are the preferred sources of the polygalactomannans, principally because of their commercial availability.
Derivatives of polygalactomannan gums are made by reacting the gums with various derivatizing agents, which react with the hydroxyl groups of the gums to form ethers or esters. For example, hydroxyalkyl ethers of polygalactomannans are made by reacting alkylene oxides with the gums as disclosed in U.S. Pat. No. 3,483,121. The galactose and mannose sugar units of polygalactomannans each have an average of three reactive hydroxyl groups. These react with alkylene oxides to produce hydroxyalkyl ethers of the gums. Each unit of alkylene oxide added to the polygalactomannan in turn adds a new hydroxyl group which is itself reactive. Theoretically, there is no limit to the amount of alkylene oxide which may be added to the polygalactomannan. As a practical matter, however, a molecular substitution (M.S.) of about 4 or 5 is a practical upper limit.
In U.S. Pat. No. 4,870,167, alkyl ethers of polygalactomannans are disclosed wherein the alkyl groups contain 1 to 4 carbon atoms which are further modified with long chain aliphatic epoxides wherein the long chains contain 8 to about 28 carbon atoms. Such polygalactomannans are particularly useful as thickening agents for aqueous systems.
Perricone et. al. U.S. Pat. Nos. 4,941,981 and 4,830,765 disclose modified non-polluting liquid phase shale swelling inhibition drilling fluids comprised of a liquid phase consisting of sea water or brine with water soluble alcohols, glycol, polypropylene glycols and the like dissolved therein. A viscosifier is added in order to suspend the solids in the aqueous phase together with a filtration control agent.
Lee et. al. U.S. Pat. No. 4,743,384 discloses a drilling fluid composition consisting of aqueous phase comprised of fresh, brine or seawater and a carboxymethyl guar gum and a metal salt selected from the group consisting of water soluble metal carbonates and bicarbonates. The drilling fluid allegedly exhibits the requisite rheological and fluid loss properties as well as excellent clay and shale inhibition with enhanced thermal stability.
In European Patent Application No. 323,627, which was published Jul. 7, 1989, polygalactomannan derivatives containing both hydrophilic substituents and hydrophilic substituents are described.
The present invention provides oil shale stabilizers which can be added to a drilling fluid to eliminate or significantly reduce shale swelling. The shale stabilizers are comprised of polygalactomannon derivatives having at least two different alkyl ether substituents, one of which is hydrophilic and the other which is hydrophobic.
The compounds of the present invention are particularly useful as shale stabilizers comprising water soluble copolymers which are essentially free of cross-linking. These shale stabilizers or inhibitors are adapted to be added to a drilling fluid which circulates through a well bore hole so that the shale stabilizers of the present invention contact and are adsorbed by the shale formation thereby inhibiting hydration of the shale thereby preventing its sloughing and collapse into the bore hole.